The present invention relates to an improvement of the structure of a fixed attenuator for the microwave band.
FIG. 1 shows a perspective view of a prior fixed attenuator for the microwave band. The attenuator comprises of a metallic mounting plate member 1 with a hole 1A, a dielectric substrate 2 mounted through an adhesive process on the metallic mounting member 1; resistance films 3 and conductors 4 applied to the surface of the dielectric substrate 2 so as to form a desired attenuating circuit of T-type or .pi.-type; and terminals 5 connected to the attenuating circuit by conductor pieces. The dielectric substrate 2 is made of for instance, alumina porcelain (Al.sub.2 O.sub.3). There have been known other fixed attenuators which do not have the metallic mounting plate.
Generally speaking when an attenuator operates, heat is generated in the resistors or the resistance films in the attenuator in proportion to the degree of the attenuation. Unless the heat thus generated is efficiently dissipated to the outside, it is difficult to produce a small size attenuator having a large power capacity. Looking at the mentioned conventional attenuators from this viewpoint, the attenuator in FIG. 1 having the mounting metallic member with a hole may meet the above requirement, except the adhesives between the metallic mounting member 1 and the dielectric substrate 2 provides the substantial heat resistance. But this conventional attenuator has drawbacks in the material cost of the mounting metallic member and in the cost of the work for joining the metallic member to the dielectric substrate. In particular, the surface of both the dielectric substrate and the metallic member must be strictly flat. On the other hand, another conventional fixed attenuator without the mounting metallic member can be manufactured at a low cost but has drawbacks in that, due to the fact that the connection and contact to the outside are made only by the connection of the conductor pieces forming the terminals, stable mounting to outside members and efficient heat dissipation can not be achieved.